The present invention generally relates to apparatus and methods of strut placement which reduces input forces on a fan. More specifically, the present invention relates to apparatus and methods of unequally spaced struts upstream from a fan where the unequally spaced struts reduce strain levels on the rotor blades of the fan.
Fan assemblies are used in many different applications. Fans are principally utilized for the movement of air for cooling or propulsion purposes, i.e. heat exchanger, air conditioning, vehicle cooling systems, ventilation and turbine engines. Fans typically consist of a hub and rotor blades where air moves through the rotor blades as the fan rotates in a circular motion. Under operation, rotor blades are subjected to various excitation forces which may vary based upon speed of operation and aerodynamic wakes that are created due to the modified air flow through the fan by any upstream struts that may be in place. Due to the excitation forces, vibratory stress and strain are developed in the blades during operation.
One method of reducing forces exerted on fan blades during operation is to place struts many chord-lengths upstream of the fan in order to smooth out the always present aerodynamic wakes. The placement of struts upstream from the fan usually include that the struts be equally spaced around the circumference of an air passage upstream from the fan. The equally spaced struts affect a fundamental excitation or forcing frequency, also known as a wake passing frequency. The wake passing frequency is the product of the total number of struts and the speed of rotation of the fan. If the wake passing frequency matches a natural resonant frequency of the blades, the blades can be driven to relatively high vibratory stress and strain which adversely affects the fatigue life of the blades. The placement of the struts upstream from the fan helps to minimize the resonant response of the fan blades and helps to ensure the avoidance of resonant crossing. Although this technique assists in reducing aerodynamic wakes and thus limits the vibratory stress and strain on the rotor blades, low excitation orders remain troublesome due to the low number of struts and the first few natural frequencies might interfere during operation. The low excitation orders place additional strain on the rotor blades and increase the likelihood of premature fatigue failure of the rotor blades.
As can be seen, there is a need for an improved apparatus and method to address low excitation orders and reduce their effect on rotor blades during operation. By reducing or eliminating potential low excitation order wakes, the vibratory stress and strain levels encountered by the rotor blades may be substantially reduced.
In one aspect of the present invention, a method of strut placement in relation to a fan assembly comprises positioning a strut arrangement in a cylindrical air passage upstream from the fan assembly, where the strut arrangement includes at least three struts; placing the at least three struts along the circumference of the air passage; and separating the at least three struts by unequal degrees of separation.
In another aspect of the present invention, a system for an air cycle machine which comprises a fan assembly; a strut arrangement, where the strut arrangement includes at least three struts, the strut arrangement is positioned upstream from the fan assembly; and a cylindrical air passage, wherein the at least three struts are separated by unequal degrees of separation along the circumference of the air passage.
In another aspect of the present invention, a strut arrangement for an air cycle machine which comprises a cylindrical air passage; and at least three struts position along the circumference of the air passage upstream from a fan assembly, wherein the at least three struts are separated by unequal degrees of separation.
In another aspect of the present invention, a method of strut placement in relation to a fan assembly comprises positioning a strut arrangement in a cylindrical air passage upstream from the fan assembly, where the strut arrangement includes three struts; placing the three struts along the circumference of the air passage; and separating the at least three struts by three unequal degrees of separation.
In another aspect of the present invention, a system for an air cycle machine which comprises a fan assembly; a strut arrangement, where the strut arrangement includes three struts, the strut arrangement is positioned upstream from the fan assembly; and a cylindrical air passage, wherein the strut arrangement lies within the air passage and the three struts are separated by unequal degrees of separation along the circumference of the air passage.
In another aspect of the present invention, a strut arrangement for an air cycle machine which comprises a cylindrical air passage; and three struts positioned along the circumference of the air passage upstream from a fan assembly, wherein the three struts are separated by unequal degrees of separation
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.